1. Field of the Invention
The present invention relates to content use management systems, content playback apparatuses, content use management methods, content playback methods, and computer programs for digital content.
2. Description of the Related Art
Recently, many users access various types of digital content including text content, such as newspapers and books, music content, and video content, such as movies, by the use of information processing apparatuses, including personal computers (PCs) and portable terminals such as Personal Digital Assistants (PDAs) and mobile phones. The available time of such digital content is restricted in some cases, just as, for example, video-rental shops and libraries allow their videocassettes and books to be available only within a limited period of time.
The available time of digital content is restricted such that, for example, digital content itself or a license for managing the digital content is assigned an available time, which is compared with the system date/time of the information processing apparatus with which the digital content is used to determine whether or not the user is entitled to the digital content.
However, since the system date/time of an information processing apparatus is accessible by the user, the user can easily override the available time by changing the system date/time. Thus, the known system has a problem in that the user can freely use digital content even if the digital content has a restricted available time.
To overcome this problem, Japanese Unexamined Patent Application Publication No. 2003-22339 proposes a content management system where reliable reference date/time information is saved in, for example, an information processing apparatus with which digital content is to be used, so that it is determined whether or not the user is entitled to the digital content on the basis of the reference date/time information and the system date/time.
According to the above-described content management system, however, if the saved reference date/time information is destroyed, it is necessary to acquire new reference date/time information from a reliable external server via a communication network to restore the destroyed date/time information. This means that if the user is in an environment where no connection to a communication network is available, the destroyed reference date/time information cannot be restored, that is, a determination cannot be made as to whether or not the user is entitled to the digital content.